Sound absorbing pipe lining having packing with different densities



Aug. 2, 1966 K. sElr-'ERT 3,263,771

SOUND ABSORBING PIPE LINING HAVING PACKING WITH DIFFERENT DENSITIES med May 21, 1964 NIVEA/TOE f/,x/f

Kuri- Seifert' 3K2/ Ww ATT YJ.

United States Patent O 3 263,771 SOUND ABSORBING PIPE LININ G HAVING PACK- ING WITH DIFFERENT DENSITIES Kurt Seifert, 36 Destouehesstrasse, Munich, Germany Filed May 21, 1964, Ser. No. 369,214 3 Claims. (Cl. 181-42) This invention relates to an absorption mufiier with a sound-absorbing system built up of heterogeneous packings for sound-bearing gas pipes such as suction pipes and exhaust gas -pipes or pipes of the character for flowing gas-oline and diesel engines or ducts of ventilation systems and air-conditioning plants. The transition from a round, oval or rectangular cross-section of the existing pipe to the narrower rectangular cross-section of the muliler channel is effected by a transition funnel.

It is known to reduce noises and channels by means of sound absorbing linings.

In practice it is mostly desired to install sound absorbing devices which are efficient on a wide frequency band, e.g. in constructional acoustics at maximum between 100 and 3200 c.p.s. or with certain types of fans between 200 and 800 c.p.s.

By adopting the known principles of design, i.e. either by using resonance dampers alone or by combining the resonance and absorption damping principles, high values of attenuation can be obtained on one or a plurality of narrow frequency bands. Since such damping devices have relatively large widths of transition steepness in their attenuation curves, there is a risk of lying already in a low attenuation range owing to actually existent slight deviations of frequency of the noises. Even if the actual noise were to remain constant over prolonged periods of time, the differences in temperature of the noise-bearing gases owing to the wavelength variations going along therewith would bec-ome perceptible by the appreciable decrease in efficiency as in the case of a frequency change. Moreover, in practice it is often desired to avoid, if possible, a frequency analysis of the noise and in such cases an approximate indication of the maximum noise frequency band is generally considered sufficient. Consequently, it is not only the question of reaching a high attenuation mean but of attaining, if possible, a continuously uniform attenuation characteristic over a given frequency range. For reasons purely concerned with absorption, porous sound absorbing material having one flow resistance only, as used in a homogeneous packing, can be laid out for the highest possible attenuation efficiency only in a determined range of frequency, e.g. the range of lower frequencies.

The object of the present invention is to improve the attenuation efficiency, such as of chamber type sound absorbing mufilers.

According to the invention, the absorption muflier consists of directly adjoining por-ous sound-absorbing materials arranged alternately behind one another and of equal or similar type, such as glass fiber, glass wool or the like, but with two different densities, whereby two corres-pondingly different resistances to flow are produced. The contact surfaces of the porous sound-absorbing materials extend normal to the flow direction of the gas and to the direction of propagation of sound -in the muffler channel. Whereas the use of sound-absorbing material with only one density is concerned with a homogeneous packing, the structure according to the present invention provides a heterogeneous packing of the soundabsorbing apparatus.

Thus, with little expenditure of structure and volume a sound absorbing device is provided, which with regard to manufacturing techniques can be produced in a simple manner. As compared with the known constructions,

ice

this sound absorbing device has a more uniform high attenuation eiiiciency in a wide range and covers, for example, the range from the low to the higher frequencies in a continuous manner.

The densities of the porous sound absorbing materials,- e.g. of glass fibres, are in the ratio of about 1:2. In the case of two farther spaced frequency bands this ratio may also change to about 1:3 or 1:4.

The contact surfaces of the packings as well as their surfaces defining the channel wall are acoustically transparent. By the proposed geometrical design, the contact surfaces of the flow resistances are substantially in the plane of the propagating wave fronts so far as these are considered plane by more or less rough approximation. Thereby, the largest possible surface and thus effective sound absorbing area is obtained for the higher frequency band unless the layered packings are not spaced too far apart. On the other hand, there must be sufficient volume for the sound absorbing material of the lower frequencies, i.e. the denser packings must not be arranged too close to each other.

An embodiment of the invention will now be described by way of example and with reference to the accompanying drawing, in which:

FIG. 1 is a longitudinal section through a sound absorbing device according to the invention,

FIG. 2 is a cross section taken on line II-II of FIG. 1, and

FIG. 3 is a longitudinal section through a sound absorbing device, according to the invention.

FIG. 1 shows a sound absorbing device 12 in the form of an elongated housing according to the invention, which has a rectangular channel cross section of a width 1 extending lengthwise thereof to be designed relatively narrow in accordance with known principles. As can be seen especially from FIG. 2, the channel is defined by side walls 2, 3 and 4. The channel side wall 2 is entirely defined, and the side walls 3 and 4 are partly defined, by the sound absorbing system proposed by the present invention, which system is outwardly limited by solid walls 13 and 14.

Referring to FIG. l, packings 6 of loose glass Wadding alternate with packings 5 of denser porous sound absorbing material. Instead of being made of loose glass wadding as the packings 6, the packings 5 may also be provided in the form of commercial factory-pressed porous sound absorbing panels having a thickness 9 of, for example, 3 cm. or more and a density of, for example, kg./m.3, which panels can be readily cut to any desired dimensions and can be easily assembled. The looser packings 6 inserted between the porous sound absorbing panels are given a certain hold by the rough surface so that caking or collapsing of `the packings 6, which otherwise would occur in the course of time, is rendered more difiicult.

Generally, two denser packings 5 are spaced apart a distance 8 substantially corresponding to packing depth 7.

The full damping effect of a packing 5 commences with the lower frequency which said packing is designed to attenuate having wave length amounting to a fifth to a sixth of the packing depth 7. This high damping effects extends to a frequency range which is dependent on the channel width 1 and limited by the beam formation effective for any type of damper.

The material of the packings 5 and 6 is retained on the inner pipe walls by an acoustically transparent covering 11, for example a fabric covering, a fine-meshed wire gauze or a perforate sheet metal plate with a backing glass web. The packing 6 may also be loosely packed in paper sacks 10. The packings 5 and 6 may be arranged consecutively in any desired number. Advantageously,

the packing 6 is pressed into the casing, whereas the packing 5 is retained between two packings 6, see FIG. 3.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

I claim:

1. In a sound absorption device for damping the full spectrum of frequencies in sound bearing gas pipes, apparatus comprising: a heterogeneous packing formed of a series of porous sound absorbing materials consisting of two groups with two different resistances -to flow directly adjoining each other in alternately arranged side-by-side relation with their contact surfaces extending normal to the uid flow direction through the sound absorption device, said packings being alternately of greater and lesser density so as t-o be responsive to, and absorb, the full gamut of frequencies passing through said device, said greater density packings occupying less volume than said lesser density packings, the width of each said greater density packing being at least five times the wave length of the lower frequencies of the sounds which said greater density packing is designed to attenuate.

2. In a sound absorption device for damping the full spectrum of frequencies in sound bearing gas pipes, the combination comprising: an elongated housing having a fluid flow passage at one side thereof; and, a heterogeneous packing formed of a series lof side-by-side, alternately arranged packings of sound absorbent material of different densities occupying the remaining area in said housing adjacent the fluid flow passage, with their mutual juncture extending normal to the Huid flow direction through said fluid ow passage, said packings being alternately of greater and lesser density so as to be responsive to, and absorb, the full gamut of frequencies passing in said housing, said greater density packings occupying less volume than said lesser density packings, the width of each said greater density packing being at least five times the wave length of the lower frequencies of the sounds which said greater density packing is designed to attenuate.

3. In a sound absorption device for damping the full 'spectrum `of frequencies in sound bearing .gas pipes, the combination comprising: au elongated housing having a uid flow passage at one side thereof; a heterogeneous packing formed of a series of side-by-side, alternately arranged packings of sound absorbing material of different densities occupying the remaining area in the housing adjacent the fluid flow passage with their contact surfaces extending normal to the uid ow direction through the lluid ow passage, said packings being alternately of greater and lesser density so as to be responsive to, and absorb, the full gamut of frequencies passing in said housing, said greater density packings Aoccupying less Volume than said lesser density packings, the width of each said greater density packing being at least five times the wave length -of the lower frequencies of the sounds which said greater density packing is designed to attenuate; and a pervious element in the housing between the uid flow passage and the heterogeneous packings to hold said packings in position.

References Cited by the Examiner UNITED STATES PATENTS Re. 22,283 3/1943 Bourne 181-48 1,433,005 10/1922 Hain 181--33 1,976,282 10/1934 Izumiyama 181-33 2,357,560 9/1944 Taforo 181---33V 3,112,007 11/1963 Ludlow et al. 181--59 FOREIGN PATENTS 1,085,907 8/1954 France. 1,138,543 1/1957 France.

733,329 7/1955 Great Britain.

OTHER REFERENCES German application, Serial No. 1,095,504 (Nordwestdeutscher Rundfunk), printed Dec. 22, 1960.

LOUIS I. CAPOZI, Primary Examiner.

LEON SMILOW, Examiner.

R. Sl. WARD, Assistant Examiner. 

1. IN A SOUND ABSORPTION DEVICE FOR DAMPING THE FULL SPECTRUM OF FRQUENCIES IN SOUND BEARING GAS PIPES, APPARATUS COMPRISING: A HETEROGENEOUS PACKING FORMED OF A SERIES OF POROUS SOUND OBSORBING MATERIALS CONSISTING OF TWO GROUPS WITH TWO DIFFERENT RESISTANCES TO FLOW DIRECTLY ADJOINING EACH OTHER IN ALTERNATELY ARRANGED SIDE-BY-SIDE RELATION WITH THEIR CONTACT SURFACES EXTENDING NORMAL TO THE FLUID FLOW DIRECTION THROUGH THE SOUND ABSORPTION DEVICE, SAID PACKINGS BEING ALTERNATELY OF GREATER AND LESSER DENSITY SO AS TO BE RESPONSIVE TO, AND ABSORB, THE FULL GAMUT OF FREQUENCIES PASSING THROUGH SAID DEVICE, SAID GREATER DENSITY PACKINGS OCCUPYING LESS VOLUME THAN SAID LESSER DENSITY PACKINGS, THE WIDTH OF EACH SAID GREATER DENSITY PACKING BEING AT LEAST FIVE TIMES THE WAVE LENGTH OF THE LOWER FREQUENCIES OF THE SOUNDS WHICH SAID GREATER DENSITY PACKING IS DESIGNED TO ATTENUATE. 